Thermodynamic Explanation of Surfactant-Free Microemulsions

There is sufficient evidence to prove that microemulsions can be formed by two immiscible liquids (generally called oil and water components) in the presence of an amphi-solvent rather than traditional surfactants, but how to explain such surfactant-free microemulsions (SFMEs) with thermodynamics is still a challenge. In this work, based on the Flory–Huggins theory, a general thermodynamic principle for SFMEs was established, by assuming SFMEs to be a pseudobinary system consisting of the water-rich and oil-rich components (i.e., the water-rich and oil-rich phases) and considering the curvature dependence of the enthalpy of dispersion between the two pseudocomponents. A new parameter, called the two-phase interaction parameter, was introduced. The thermodynamic model can predict the SFME region in the ternary phase diagram as well as the droplet size and type of SFMEs formed. The formation and stability of SFMEs are attributed to the balance between the entropy and enthalpy of dispersion of the two phases. The rationality of the thermodynamic principle suggested here was confirmed by the experimental results of the ternary mixture of n-butanol (oil), ethanol (amphi-solvent), and water. This work provides a thermodynamic explanation for SFMEs, which can deepen our understanding of the nature of SFMEs.